The fungus Aspergillus japonicus MF275 produces himeic acid A (1), containing a 4-pyrone ring, along with its congeners, himeic acids B (2) and C (3). During culture, 1 was gradually converted to 3, the corresponding 4-pyridone derivative. A study of the relationship between the culture pH and the fungal metabolites showed that a decrease from pH 6.5 to pH 2 is essential for production of 1, while a subsequent increase to pH 5 is necessary for production of 3. In addition, we revealed that 1 was non-enzymatically converted to 3 by the incorporation of an ammonium nitrogen atom in a pH 5 buffer, and that 1 was converted to 2 at a conversion ratio of 50% during incubation in MeOH for five days.

日本曲霉MF275真菌会产生带有4-吡喃酮环的heeic acid A（1），以及它的同类物，hemicic acid B（2）和C（3）。在培养过程中，1逐渐转化为3，即相应的4-吡啶酮衍生物。对培养液pH和真菌代谢物之间关系的研究表明，从pH 6.5降低到pH 2对于产生1是必不可少的，而随后将pH升高到5则对于产生3是必不可少的。此外，我们发现1被非酶转化为3通过在pH 5缓冲液中引入铵氮原子，在MeOH中孵育5天后，以50％的转化率将1转化为2。